In long distance fiber optic communication systems it is important to monitor the health of the system. For example, monitoring can be used to detect faults or breaks in the fiber optic cable, faulty repeaters or amplifiers or other problems with the system.
Prior art monitoring techniques include the use of a testing system which generates a test signal and modulating the test signal onto a single channel (or wavelength) with the transmitted data signal. For example, the data signal may be amplitude modulated by the test signal. A loop-back coupler within an optical amplifier or repeater located downstream is used to return a portion of the transmitted signal (data signal plus test signal modulation) to the testing system. The testing system then separates the test signal from the data signal and processes the test signal to examine the health of the transmission system. U.S. Pat. Nos. 4,586,186 and 4,633,464 to C. Anderson et al. discloses a similar technique to modulate test response information from a repeater onto the main data signal to monitor the health of the system.
There are several disadvantages and drawbacks, however, with the prior art monitoring systems. Due to the low signal to noise ratio of the returning test signal, measurements must be taken over a significant period of time. To properly assess the status of the communication system requires that data be collected over periods ranging from about a half hour when the system is out-of-service to about eight hours when the system is in-service. Therefore, if the monitoring system is to detect a fault, the fault must be manifest over the time period that the data is collected. Accordingly, the previously mentioned line-monitoring techniques are most effective in locating static faults. Intermittent faults, however, will go undetected if their duration is substantially less than the time over which monitoring statistics are collected.
Therefore, there is a need for a line monitoring system that can locate intermittent as well as static faults.